This is the 4372nd most frequent Russian word.
бухгалтерской
"Бухгалтерской" means "accounting" or "bookkeeping" (adjective form).
Here, 'бухгалтерской' refers to something related to accounting (adjective form) and is describing 'отчетности' (reporting).
Я приобрел новую книгу по бухгалтерской отчетности.
I purchased a new book on accounting reporting.
In this sentence, 'бухгалтерской' modifies 'школе' (school), indicating that the school specializes in accounting.
В бухгалтерской школе я изучал основы экономики.
In accounting school, I studied the basics of economics.
'Бухгалтерской' pertains to 'фирмы' (firm), denoting that the firm operates in the accounting domain.